System Volume
The system volume refers to the disk volume that contains the hardware-specific files that are needed to start Windows, such as Ntldr, Boot.ini, and Ntdetect.com.

On computers that are running the Intel x86 line of CPU processors and later versions, the system volume must be a primary volume that is marked as active. This requirement can be fulfilled on any drive on the computer that the system BIOS searches when the operating system starts.

The system volume can be the same volume as the boot volume. However, this configuration is not required.

and

Boot volume
The boot volume refers to the disk volume that contains the Windows operating system files and the supporting files. By default, the Windows operating system files are in the WINDOWS folder, and the supporting files are in the WINDOWS\System32 folder.

The boot volume can be the same volume as the system volume. However, this configuration is not required.

There is only one system volume. However, there is one boot volume for each operating system in a multiboot system.

So… the “system volume” is the volume that contains the boot files and the “boot volume” is the volume that contains the system files. It might have been opposite day when this was named. Yikes.